MEXICO CITY (AP) — Police have found five charred bodies inside a pickup truck abandoned and set ablaze on a northern Mexico highway.

An official with Coahuila state prosecutors says the five men found Thursday died of asphyxiation and all had been shot in the head post-mortem.

The official spoke on condition of anonymity because he is not allowed to discuss an ongoing investigation.

Police found the victims ages 15 to 25-years-old outside the capital city of Saltillo, an area where the Zetas drug cartel is active.

Authorities are investigating whether the attack is related to this week’s capture of top Zetas leader Miguel Angel Trevino Morales. Experts have warned his arrest could cause a spike in violence in northeast Mexico, an area the brutal cartel dominates.